Optical fibres have many characteristics such as low attenuation, wide frequency band and low weight, and are economically used in submarine cables for a long-distance transmission of large amounts of information.
FIG. 14 illustrates a sectional view of a submarine optical fibre cable. An optical fibre unit 1 is protected from high water pressure by a pressure-resistant metal sheath 2 which is used as an electrical power feeding conductor as well. The metal sheath 2 is composed of three units of fan-shaped segments 2a, 2b and 2c disposed longitudinally along the optical fibre. Spaces 3 between the segments are filled with a compound. Tension wires 4 are wound on the outer surface of the pressure-resistant sheath 2. A metal tube 5 is provided to hold the tension wires. A cover 6 is made of plastic insulating material and is sometimes covered by an exterior material to prevent cable damage. All the spaces associated with those components as described above are filled with the compound to prevent water from entering the metal.
The use of the segments 2a, 2b and 2c in the pressure-resistant sheath is described in Japanese Post-Exam Publication No. 7361/1984. According to the specification of the Publication, the pressure-resistant sheath 2 can be made thick enough to protect the optical fibre unit 1 from a high water pressure because the sheath 2 is composed of fan-shaped segments 2a, 2b and 2c disposed longitudinally along the optical fibre unit 1. In addition, since no heat treatment is necessary for the production of the pressure-resistant sheath, the transmission characteristics of the optical fibre are not impaired at all. Furthermore, the sheath itself can function as a tension member as well.
However, the metal segments 2a, 2b and 2c to form a pressure-resistant sheath 2 must be highly polished to provide intimate contact among the segments. No difference in the level of the contact faces shall be allowed. Since the metal segments 2a, 2b and 2c are only disposed longitudinally along the optical fibre, bending stresses often cause a gap between these segments to result in water ingress into the pressure-resistant sheath 2 even if the inside of the sheath 2 has been filled with a compound. In particular, the pressure-resistant sheath 2 is very weakly bonded to the optical fibre unit 1 because high lateral pressures cannot be applied to the optical fibre unit. Thus, water is likely to penetrate into the contact faces of the unit 1 and the sheath 2 as well as the segments 2a, 2b and 2c.
Thus, the object of this invention is to provide a pressure-resistant sheath for submarine optical fibre cables wherein the adhesion force associated with the pressure-resistant sheath 2 and the segments 2a, 2b and 2c is so strengthened that the optical fibre unit 1 can be thoroughly protected from various stresses imposed at the stage of manufacturing and aging experienced after manufacturing.
Another object of the present invention is to provide a method for the production of a pressure-resistant sheath for a submarine optical fibre cable as described above.